I'm Cummins swapping a '74 Duster, thought I'd share the progress.

How about a mid 70’s Camaro stub? Close on track width,

front steer(more room in the back for down pipe, could build a shallow but longer oil pan). Put impala splindles and hubs on (5 on 5 bolt pattern, same as c10 pickups of that era, 5 1/2 springs are very common in racing so good choice of springs).
Run 2-3 tubing straight back to whatever rear suspension. Good cage to tie everything together front to back!
I would slide that thing back always, you’re gonna have to hack the firewall anyway, give you room for a good radiator.
Don’t know anything about that tranny, the 2nd gen Cummins would need a better clutch at like 500 horsepower, not sure about the tranny of the era. The g trag and 4500 both have a case iron case, they are not light, I think I have a 2 wd gtrag in the barn.
The crown Vic’s run a perimeter frame so you would have to channel the heck out of both sides to make it look decent, but would be a good option also.
Might need a plastic toboggan for a hood scoop, I like the way you’re thinking!!!
I really think this front stub might work, way stronger than about any Mopar front end.